swig: add test

Closes Homebrew/homebrew#35493.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/swig.rb

@@ -1,5 +1,3 @@
-require "formula"
-
 class Swig < Formula
   homepage "http://www.swig.org/"
   url "https://downloads.sourceforge.net/project/swig/swig/swig-3.0.2/swig-3.0.2.tar.gz"
@@ -21,6 +19,30 @@ class Swig < Formula
     system "./configure", "--disable-dependency-tracking",
                           "--prefix=#{prefix}"
     system "make"
-    system "make install"
+    system "make", "install"
+  end
+
+  test do
+    (testpath/"test.c").write <<-EOS.undent
+      int add(int x, int y)
+      {
+        return x + y;
+      }
+    EOS
+    (testpath/"test.i").write <<-EOS.undent
+      %module test
+      %inline %{
+      extern int add(int x, int y);
+      %}
+    EOS
+    (testpath/"run.rb").write <<-EOS.undent
+      require "./test"
+      puts Test.add(1, 1)
+    EOS
+    system "#{bin}/swig", "-ruby", "test.i"
+    system ENV.cc, "-c", "test.c"
+    system ENV.cc, "-c", "test_wrap.c", "-I/System/Library/Frameworks/Ruby.framework/Headers/"
+    system ENV.cc, "-bundle", "-flat_namespace", "-undefined", "suppress", "test.o", "test_wrap.o", "-o", "test.bundle"
+    assert_equal "2", shell_output("ruby run.rb").strip
   end
 end